Gold and gold alloys have been used for centuries for dental restorations and jewelry, but the price of gold in today's market imposes economic constraints on the continued use of these materials. Until this invention, the two known ways of obtaining a gold color in an alloy involved the use of sufficient gold or copper to provide the color, these being the only metallic elements which have a yellow to red color.
The traditional yellow alloys for dental restorations and jewelry have been gold-copper-silver with greater use of silver in dental alloys. Copper tends to enhance the gold color unless used in excess, when it enhances the red. Silver enhances the yellow, with excess enhancing a greenish hue. Copper base alloys, notably beta brass and aluminum bronze, have been able to reproduce the color of gold alloys, but have not overcome the tarnish and corrosion susceptibility of copper.
The American National Standards Institute (ANSI), through its Accredited Standards Committee (ASC) MD156, which is sponsored by the American Dental Association (ADA), has developed ANSI/ADA Specification No. 5 which addresses the mechanical properties required for alloys for dental restorations. ANSI/ADA Specification No. 5 for many years has included four types of casting alloys in common use in dentistry. Type I, for restorations subject to very slight stress such as some inlays, having 0.2% offset yield strength (0.2%YS) of up to 140 MPa (20,000 psi); Type II, for restorations subject to moderate stress, such as inlays and onlays, having a 0.2%YS between 140 MPa and 200 MPa (29,000 psi); Type III, for restorations subject to high stress, such as onlays, crowns, thick vaneer crowns and short-span fixed partial dentures, having a 0.2%YS between 200 MPa and 340 MPa (49,000 psi); and Type IV, for restorations thin in cross section and subject to very high stress, such as thin veneer crowns, long-span fixed partial dentures and removable partial dentures, having a 0.2%YS of at least 340 MPa, and hardenable to at least 500 MPa (72,000 psi). Types III and IV are of greater significance here. These are the ones that are most frequently used in today's dentistry. Although the strength of Type III alloys is greater than required for Types I and II applications, it is usually used for these application to avoid the necessity of stocking additional alloys.
This invention employs the term dental alloy within the context of the above-described standards, and it should be understood that any dental alloy of this invention will meet at least the strength and hardness requirements of ANSI/ADA Specification No. 5 for Type II, and almost always for Type III as well.
Many dental alloys have been developed by the instant inventors and others to reduce alloy intrinsic cost, but all such alloys possessed certain disadvantages. If gold was eliminated or substantially reduced without the addition of copper to produce the gold color, the alloy was gray (usually called white). If the copper was increased enough to maintain the color in the absence of gold, the alloy lost its corrosion and/or tarnish resistance. In some instances, in addition to losing color the alloys were not strong enough to perform the required function as indicated in ANSI/ADA No. 5.
In recent years the acceptance of "white" alloys has increased. However, there still remains a significant need within the dental and jewelry industries for a gold colored alloy which is economical to produce and which complies with the ANSI/ADA standards set forth above.